1. Field of the Invention
This invention relates to a method for the production of a heat exchanger. More particularly, it relates to a method for the production of a heat exchanger having a core part thereof brazed by the use of a flux of potassium fluoroaluminate complex (K.sub.3 AlF.sub.6 and KAlF.sub.4, hereinafter referred to NOCORC flux), which heat exchanger core is made of an aluminum material and therefore is enabled to obviate the necessity for application of a coating subsequent to the brazing.
2. Description of the Prior Art
In recent years, the heat exchangers such as radiators for automobiles and condensers for car coolers use an aluminum material (aluminum or aluminum alloys) in their core parts for the purpose of reduction in weight.
In the heat exchanger made of an aluminum material, used as a condenser in a car cooler, for example, the core part thereof is produced by molding its heat transfer fins with an aluminum material clad with a brazing filler and its tube with an aluminum material, fitting the components to each other with the aid of a jig, for example, and brazing them. As is widely known, in the brazing of parts of an aluminum material, an aluminum oxide coating of a high melting point formed on the surface of the aluminum material is required to be removed from the positions destined to undergo brazing. Unless the brazing is carried out under special conditions in a vacuum or in an atmosphere of inert gas (inevitably by the use of an expensive apparatus), there is followed a practice of applying a flux for the removal of aluminum oxide at least on the positions expected to undergo brazing in advance of the brazing. The flux heretofore used for this purpose has been a mixture of inorganic chlorides (such as for example, alkali metal chlorides and alkaline earth metal compounds). Since such water-soluble compounds by nature are liable to corrode aluminum, however, the core part of the heat exchanger assembled by brazing has inevitably to be subjected to a cleaning work for the removal of flux residue subsequently to the brazing.
Recently, to take the place of the conventional flux of inorganic chlorides, NOCOLOK flux (Registered trademark of Alcan Research and Development Limited.) (potassium fluoroaluminate complex) has been proposed and adopted for actual use. This NOCOLOK flux possesses a characteristic property of remaining non-hygroscopic prior to brazing and becoming substantially water-insoluble subsequently to brazing and, at the same time, another characteristic property of assuming, while in a fused state, reactivity at a temperature below the melting point of the brazing filler and consequently functioning for aluminum oxide and remaining unreactive to aluminum (Japanese Patent Publication No. SHO 58(1983)-27,037). When the brazing is effected by the use of NOCOLOK flux, therefore, the phenomenon of the corrosion of aluminum by the residual flux experienced by the conventional flux of inorganic chlorides is substantially elminated.
Under the belief that application of a coating to a heat exchanger solely for improving the glare-proofing property and commercial value of the heat exchanger is very uneconomical, efforts have been devoted to development of a heat exchanger which can dispense with the coating work of such uneconomical and time-consuming nature. So far, there has been proposed a heat exchanger of the kind having a core part thereof formed by joining a tube of aluminum material and heat-transfer fins of aluminum material with a solder, which heat exchanger is characterized by the fact that at least part of the surface of the core part is coated with an inorganic sintered layer possessing a phase of the two complexes of pigment-containing potassium hexafluoroaluminate (K.sub.3 AlF.sub.6) and potassium tetrafluoroaluminate (KAlF.sub.4) (U.S. Ser. No. 765,385).
To be specific, this heat exchanger has originated in a new knowledge that when the flux made of a mixture of potassium hexafluoroaluminate and potassium tetrafluoroaluminate (NOCOLOK flux) is used as the flux during the brazing work, there is formed in consequence of the brazing, on the surface of aluminum material coated with this NOCOLOK flux, a water-insoluble, rigid inorganic sintered layer possessing a phase of two complexes of potassium hexafluoroaluminate and potassium tetrafluoroaluminate. Thus, by adding a pigment to the NOCOLOK flux thereby enabling this pigment to be eventually incorporated in the inorganic sintered layer to be formed on the surface of the aluminum material, there is formed a colored inorganic sintered layer capable of reducing the phenomenon of glare.
When the method which causes the pigment to be incorporated in the inorganic sintered layer by being added to the NOCOLOK flux is employed in the manufacture of the heat exchanger of the type under discussion, since the pigment such as carbon black is generally insoluble in an aqueous solution to be used as a dispersant for the NOCOLOK flux, required dispersion of the pigment in the flux of slurry constitution inevitably entails the work of stirring and the produced colored inorganic sintered layer has the possibility of suffering form uneven coloration.
This invention, therefore, is aimed at eliminating the drawbacks mentioned above.
To be specific, this invention has as an object the provision of a method for the production of a heat exchanger having a core part thereof soldered with the NOCOLOK flux, which method enables a heat exchanger possessing a sufficiently high glare-proofing property to be easily produced with high commercial value without requiring application of a coat to the surface thereof.